The RF amplifier is an important component of a mobile communications handset such as a mobile phone. The increasing popularity of mobile phones, and the developments in the industry, such as the provision of multiple functions in mobile phones and the ever increasing number of subscribers, places great demands on the design of the mobile phone.
Key design features in a RF amplifier for a mobile phone are power consumption and linearization. In particular, battery power and battery life is a constant issue in mobile phones and power-effective solutions are constantly needed. This is particularly so with the incorporation into mobile phones of other functions such as cameras and the use of large LCD displays. The incorporation of these other functions and features into mobile phones also places great demands on space on the printed circuit board. Furthermore, the linearity of the input-output characteristics of the RF amplifier is very important to minimize adjacent channel interference (which can cause crosstalk in cellular networks). The ideal RF amplifier in a mobile phone would therefore be highly efficient, power effective, with good linearity characteristics, and would occupy minimum space on the printed circuit board. Often, however, there is a trade-off between linearity and power efficiency.
FIG. 1 shows a conventional RF amplifier circuit. Under large signal conditions, the base-emitter voltage (VBE) of the transistor will drop with the increase in the rectified DC current and the decrease of the transconductance resulting in severe non-linear distortion in signal amplitude.
Various linearization techniques have been proposed to overcome or mitigate this problem. These include harmonic feedback, feed-forward, base-band feed-forward, and pre-distortion techniques. Each of these prior techniques has its own advantages and disadvantages.
For example, generally speaking, harmonic feedback from the output port can reduce intermodulation distortion (IMD) products by the order of 20 dB, but it can also cause serious gain reduction and instability problems.
Feed-forward linearization provides good distortion improvement and broadband operation, but requires complex, bulky and expensive control circuits which prevent the full use of active devices and limits the use if this technique to the base-station level. Even after a number of years of development the typical efficiency of feed-forward techniques is only about 8% at best and this figure is unlikely to be improved upon in the near future.
Pre-distortion techniques can achieve a much higher efficiency of about 15% and linearization using series or parallel diode pre-distorters offers a relatively compact and low-cost approach to improving the linearity of the RF amplifier. However, a suitable match of the non-linear transfer characteristics from the pre-distorters is always required and in practice limits the degree of cancellation to below 10 dB.
More recently several methods have been proposed based on the exploitation of second-order signals. These techniques are based on an additional injection of either the second-order harmonic or difference frequency signals to interact with the fundamentals and can give more than 25 dB in cancellation. However, precise gain and phase adjustments are still required which can make these approaches impractical. In addition, since auxiliary predistortion circuitry is needed there is an increase in complexity and a reduction in efficiency.